1. Field of the Invention
The present invention relates to an apparatus for selectively measuring ions in a liquid and more specifically to a protection system for protecting an ion sensor of the apparatus. The apparatus comprises a measuring circuit including a chemically selective ion sensor in the form of an ion-sensitive field effect transistor (ISFET), a reference electrode, and an amplifier.
1. Description of the Prior Art
Measuring apparatus for measuring ions in a liquid using an ISFET has been previously proposed. See for example the Johnson et al. U.S. Pat. No. 4,020,830. Such an apparatus includes a measuring circuit and an ISFET immersed in a liquid for selectively measuring the ion activity in the liquid. Different ISFETs are used for measuring different ion activities, such as, for example, pH, pK and pNa.
Although the use of an ISFET in such a measuring apparatus has been found to be very useful for measuring ions, especially in the medical and biomedical field, such usefulness has some limitations in actual practice. In this respect, voltages that develop between the surface of the gate insulator of the ISFET and the underlying bulk material of the ISFET establish an electrical field in the gate insulator which may adversely affect the operation of the ISFET.
If this electrical field, which may be caused by external influences, exceeds a maximum value, dielectriccal breakdown may occur in the gate insulator region and the ISFET may be damaged. When the gate insulator consists of a multilayer system (for example of combinations of SiO.sub.2 --Si.sub.3 N.sub.4 or SiO.sub.2 --Al.sub.2 O.sub.3), an unduly strong electrical field may produce still another effect, namely a shift in the threshold voltage of the ISFET, which may be permanent or which is only corrected after a long period of time. This results in the apparatus being permanently or temporarily out of order since the calibration of the apparatus is now incorrect.
Accordingly, it is desirable to provide some form of protection structure or circuitry to prevent the establishment of such electrical field or to prevent the calibration of the apparatus from being disturbed or upset by the electrical field. In this respect, an electrical field may be created by external influences or effects such as, for example, from electrosurgical apparatus, heart defibrillation apparatus, electromagnetic influences resulting from switching inductive elements on or off, and also from electrostatic voltages generated during the manufacture of the ISFET sensor or during use of the apparatus in practice.
It has heretofore been proposed to take steps to protect ISFET sensors from damage due to the presence of high voltages. See for example the Thesis of Rosemary Smith entitled "Ion Sensitive F.E.T. with Polysilicon Gates" at the University of Utah, June, 1982. This publication proposes the application to the gate insulator of a conductive polysilicon layer which, via a Zener diode or a MOSFET switch is connected to the source electrode of the ISFET. The polysilicon layer should in turn be provided with an ion sensitive layer of material to be contacted with the liquid to be investigated. It has been found, however, that the proposed protection system limits the application of the sensor to situations where only fast changes in ion activity are to be monitored.
It has also been proposed to provide a protection circuit for a metal-oxide semi-conductor field effect transistor (MOSFET) which includes a MOSFET. See the Yoshida et al. U.S. Pat. No. 4,086,642.
Further, it has been proposed to provide a silicon, PN junction surge current suppressor for protecting electronic circuitry. See U.K. patent application No. 2060255.
ISFETS, however, have no metallic gate electrode, in a way similar to a MOSFET, to which a protective element can be connected.